FIELD OF THE INVENTION
This invention relates to a method of manufacturing a prepreg in which reinforcing fibers are impregnated with a matrix resin and particularly to a prepreg manufacturing method suitable for production of a thick prepreg.
Presently, fiber reinforced composite materials in which reinforcing fibers are hardened together with a matrix resin, have been widely used in various technical fields such as automobiles, airplanes, building materials, sporting goods, etc. Prepregs have extensively been used for manufacturing such fiber reinforced composite material products.
Such a prepreg consists of a flexible sheet in which a fiber reinforced composite resin layer having reinforcing fibers impregnated with a matrix resin is formed in a thin layer on a release paper as a support and then the matrix resin is partially cured in advance. The prepreg, in which reinforcing fibers are preliminarily impregnated with the matrix resin, is covered with a cover film thereon to make it a product, and is stored for later use.
In order to produce the fiber reinforced composite material product by using the prepreg, for instance, a desired number of the prepreg sheets are laminated on a product model. The prepreg laminates are subjected to pressing and heating not only to shape them but also to cure the matrix with the result that the composite material product is easily obtained.
Heretofore, such a prepreg has been manufactured in the following process.
A unidirectional fiber reinforced prepreg will be described by way of example. A matrix resin is coated on a first release paper. The first release paper coated with the matrix resin and a second release paper on which there is no matrix resin coat, are successively fed in between a pair of rollers. Simultaneously, reinforcing fibers are successively supplied in the longitudinal direction of the fibers in between the rollers covering the width of the release paper. The first and second release papers are superimposed and pressed together in such a manner that the reinforcing fibers are unidirectionally arranged on the matrix resin layer. Subsequently, the reinforcing fibers and the matrix resin layer are pressed and heated at the following press and heat section where press rollers and a hot plate are disposed, so that the reinforcing fibers are forced into the matrix resin layer and impregnated with the matrix resin. Simultaneously, the resin is partially cured. Thus, a unidirectional fiber reinforced prepreg is obtained, in which the unidirectionally arranged reinforcing fibers are impregnated with the matrix resin. The obtained prepreg is taken up on a roller after separating the second release paper from the prepreg and then a cover film is applied onto the prepreg to make it a product.
Alternatively, the second release paper, in addition to the first release paper, is coated with the matrix resin. A unidirectional fiber reinforced prepreg in which the unidirectionally arranged reinforcing fibers are impregnated with the matrix resin, is obtained by using the first and second release papers coated with the matrix resin, and impregnating reinforcing fibers with the matrix resin, and then partially curing the resin according to the process described above. Similarly, the obtained prepreg is taken up on a roller after separating one of the release papers from the prepreg and a cover film is applied onto the prepreg to make it a product.
On the other hand it is necessary that the matrix resin should be coated on the release paper in a uniform thickness. Further, from the viewpoint of operational efficiency in uniformly coating the matrix resin it is necessary that the matrix resin should have a viscosity in a predetermined low viscosity range (Vis). For example, a viscosity between 2,000 to 20,000 cp (centipoise) is recommended during coating operation, as shown in FIG. 5 which schematically shows a viscosity-temperature property of the matrix resin. This is particularly required in the case where the matrix resin is thickly coated to manufacture a thick prepreg.
Formerly, the matrix resin had to be used at a high temperature (Ta) for instance, at a temperature between 100.degree. and 110.degree. C. in order to conduct coating operation in the predetermined viscosity range (Vis) since the viscosity of the matrix resin used is relatively high. This increases the constructural burden by requiring a resin temperature maintaining means as part of the coating system.
Therefore, attempts have been made to conduct coating by using the matrix resin which has a relatively low viscosity, the property of which is shown by the straight line B. In this case, the viscosity range (Vis) for coating operational can easily be obtained even if the temperature of the resin is reduced from the temperature (Ta) to a relatively low temperature (Tb), for instance, a temperature between 70.degree. and 80.degree. C.
According to the aforesaid process, the construction of the resin temperature maintaining means of the coating system is made easier. Furthermore the obtained prepreg has reinforcing fibers which are sufficiently and very uniformly impregnated with the matrix resin, since the matrix resin itself has a low viscosity.
However, even if the matrix resin layer into which the reinforcing fibers are embedded is partially cured after the reinforcing fibers are impregnated with the matrix resin, the matrix resin layer exhibits a lower viscosity to the extent that the relatively low viscosity matrix resin is used. Therefore, the matrix resin layer of the prepreg is sticky, so that the cover film applied to the resin layer adheres to the same. This causes disadvantages in that when the fiber reinforced composite material is manufactured, it is difficult to separate the cover film, resulting in interference with the use thereof.